1. Field of the Invention
The present invention relates to a heat-dissipating structure of an electronic part mounted to a circuit board in a shield case.
2. Description of the Related Art
In an electronic device forming a high-frequency circuit, a circuit board with an electronic part mounted thereto is usually encased in a shield case. When the electronic part mounted onto the circuit board is one that generates heat, such as an integrated circuit (IC), part of the shield case is formed so as to contact the body of the electronic part that generates heat, allowing dissipation of heat by conduction through the shield case. A description will now be given of a conventional heat-dissipating structure of such a type, with reference to FIGS. 7 and 8. FIG. 7 is a sectional view of the main portion of the shield case, illustrating the conventional heat-dissipating structure. FIG. 8 is a plan view of the cover in the conventional heat-dissipating structure.
A circuit board 2 is encased in a shield case 1. An electronic part 3, such as an integrated circuit (IC), a power transistor, a resistor, or the like, is formed on the circuit board 2 in order to form a high-frequency circuit. When the electronic part 3 is one generating heat, such as an IC or an electronic part handling a large amount of electrical power, part of a cover 5 of the shield case 1 is cut and bent to form a contact piece 6 which contacts the electronic part 3 (IC), allowing the heat from the electronic part 3 to be dissipated, by conduction through the entire shield case 1, through the contact piece 6.
The contact piece 6 is formed by cutting part of the cover 5 into a C shape, and bending the cut portion inwardly of the shield case 1 such that it contacts the electronic part 3, with an end 7 of the bent portion positioned parallel to and thus being in uniform contact with the upper surface of the body of the electronic part 3. Silicone grease 8, which is an adhesive-like substance with good heat conductivity, is applied between the end 7 and the upper surface of the body of the electronic part 3 to increase heat dissipation efficiency.
In the above-described conventional heat-dissipating structure, however, the contact piece 6 is bent inwardly of the shield case 1 in order to bring it into contact with the electronic part 3, so that a hole 9 as large as the contact piece 6 is formed in the cover 5. Therefore, when the electronic device is one forming a high-frequency circuit, a high-frequency signal in the shield case 1 travels out through the hole 9, resulting in unnecessary emission and leakage of signals to the outside; or a high-frequency signal from an external electronic device rushes into the shield case 1 from the hole 9, and interferes with the operation of the electronic device.
There are some currently-used high-frequency electronic devices which incorporate a microcomputer. In these electronic devices, it is not possible to keep clocks in a normally-operating state. A possible solution to this problem is to form a smaller hole 9 in the cover 5. In order to make the hole 9 smaller, however, the width of the contact piece must be made narrower, which reduces the efficiency with which heat can be conducted, so that there is a limit to how small the contact piece 6 can be made. Accordingly, the hole 9, formed in correspondence with the size of the contact piece 6 that prevents a reduction in heat conduction, is covered with a conductive tape in the final step of producing the electronic device. This, however, increases the number of production steps and component parts (due to the use of the conductive tape), making the electronic device more costly. Therefore, the a present invention aims at achieving a heat-dissipating structure having a high heat conductivity and capable of reducing the amount of interfering signals emitted from or rushing into the shield case.